While solar radiation is a source of thermal energy of a high temperature at its origin, the use of the same under the conditions of the flow which reaches the surface of the earth destroys practically all its potential for conversion into work due to the drastic reduction in the temperature available in the flow. For this reason, thermoelectric solar plants and optical concentration systems are used, which achieve a greater density of the flow and thus higher temperatures.
Currently, there exist three main different technologies developed for use in Solar Facilities, these being: the central receiver type, cylindrical-parabolic collectors and Stirling discs. All of these make use only of the direct component of solar radiation, which makes it necessary for them to feature solar tracking devices:
1. Central receiver-type systems (3D) use mirrors of a large surface area (40-125 m2 per unit) called heliostats, which feature a control system in order to reflect the direct solar radiation onto a central receiver located at the top of a tower. By means of this technology, the concentrated solar radiation heats a fluid in the interior of the receiver to a temperature of as much as 1000.degree. C.; this thermal energy may subsequently be used for the generation of electricity.
2. Regarding cylindrical-parabolic collectors (2D), the direct solar radiation is reflected by cylindrical-parabolic mirrors which concentrate the same onto a receiving or absorbing tube through which there circulates a fluid which is heated as a consequence of the concentrated solar radiation which falls on the same at a maximum temperature of 400.degree. C. in this way, the solar radiation is converted into thermal energy which is used subsequently for the generation of electricity by means of a Rankine water/steam cycle.
A variation in this technology is embodied in linear Fresnel concentration systems, in which the parabolic mirror is replaced by a Fresnel discretization with mirrors of smaller dimensions, which may be flat or may feature a slight curvature at their axis, and by means of controlling their axial orientation allow the concentration of solar energy on the absorbing tube, which in this type of applications usually remains static.
3. Parabolic Stirling disc systems (3D) use a surface area of mirrors installed on a parabola of revolution which reflect and concentrate the rays of the sun onto a focal point, at which the receiver is located; in said receiver the working fluid of a Stirling engine is heated; this engine in turn operates a small electrical generator.
In central-receiver systems, water-steam technology is currently the most conventional. The steam is produced and superheated in the solar receiver at temperatures of approximately 500.degree. C. and 10 MPa (100 bar) and is sent directly to the turbine. In order to reduce the impact of transitional conditions (the passing of clouds, etc.) a storage system is used (melted salts or a rock/oil thermocline). This concept was the first to be tested due to its permitting the transposition of the habitual techniques of thermal power plants and to its permitting the direct access of the steam issuing from the solar receiver into the turbine.
The use of superheated steam may allow the implementation of thermodynamic cycles of a higher efficiency in power plants.
The difficulty of solar technology for the production of superheated steam lies in the demanding conditions of temperature at which the receiver must operate. The walls of its pipes are continuously subjected to thermal cycles between ambient temperature, the temperature of the steam with which the receiver is fed (250 to 310.degree. C.), and the temperature necessary at its wall for the generation of superheated steam at 540.degree. C. or nearly 600.degree. C. Unlike the receivers which generate saturated steam, which operate at a temperature which is almost uniform throughout their sections (330.degree. C.), superheated steam receivers increase the temperature of their pipes in accordance with their greater proximity to the steam outlet zone.
The difficulties encountered in the experiments carried out on superheated steam receivers in the eighties were centered on two main aspects:
A lack of controllability of the system, especially when faced with transitional conditions, the passing of clouds, etc., due mainly to the bad thermal properties of the superheated steam. In both receivers, the most frequent structural fault was the appearance of cracks. The thermal tension due to the great differences in temperature brought about the appearance of cracks in the interstitial weld between subpanels. This situation occurred fundamentally during downtime, when the water in a subpanel, at saturation temperature, flowed upwards, where the temperature was still that of the superheated steam, while in the adjacent subpanel this phenomenon did not occur
The problem of operating at high pressures, which made necessary greater thicknesses of the walls of the piping; this, when the time came to transfer high densities of power to the heat-carrying fluid, necessarily implied high thermal gradients.
The invention proposed below therefore deals with the agglutination of the advantages of the use of high-temperature steam, resolving the existing risks, achieving a greater control over the plant and thus favouring the stability and durability of the same.